The present invention relates to a direction finder for use in a navigation system of a vehicle, which is capable of finding a moving direction of the vehicle on the basis of terrestrial magnetism.
There have been many navigation systems of this type and they are based on a principle which is shown in FIGS. 1 and 2. In FIG. 1, a horizontal component H of terrestrial magnetism, which is referred to as "terrestrial magnetism H" hereinafter, is detected by a terrestrial magnetism sensor 2 mounted on the vehicle 1, e.g., an automobile, whose heading makes an angle .theta. with respect to the direction of terrestrial magnetism H, i.e., north. That is, the sensor 2 detects a field component H.sub.y (=cos .theta.) of the magnetism H which is in parallel to the moving direction A of the vehicle 1 and a field component H.sub.x (=sin .theta.) orthogonal to the direction A and provides electric signals x and y in the form of, for example, voltage signals corresponding thereto. The electric signals x and y are amplified suitably. Thus, the signals x and y can be expressed by EQU x=KH.sub.x =KH sin .theta. (1a) EQU y=KH.sub.y =KH cos .theta. (1b)
where K is a magnetism to voltage conversion coefficient.
The detected signals x and y when the field components H.sub.x and H.sub.y are zero are calibrated to zero so that the magnitudes of the signals x and y are in proportion to the intensities of the components H.sub.x and H.sub.y, respectively, and can be used as reference values, as shown by the above equations.
FIG. 2 shows a x-y coordinates system on which points each defined by a pair of electric signals x and y are plotted. A locus of the plot, when the vehicle 1 moves around once, describes a circle 1 and the angle .theta. between the moving direction A of the vehicle 1 and the terrestrial magetism H becomes as shown in FIG. 2. Therefore, it is given by EQU .theta.=tan.sup.-1 (x/y) (2)
The direction of the terrestrial magnetism H is not always coincident with the geographic north and there is an error, i.e., declinatiin therebetween. The declination depends on a geographical area of the earth. In this description, however, it is assumed that there is no declination for simplicity of explanation.
It has been known that, due to magnetization of magnetic material forming various components of the vehicle, the angle .theta. calculated according to the equation (2) is not always correct.
Describing this in greater detail with reference to FIGS. 3 and 4, the vehicle 1 is subjected to a magnetic field H.sub.v shown in FIG. 3 produced by those magnetized components. With the magnetic field H.sub.v, the magnetic field to be detected by the terrestrial magnetism sensor 2 becomes a composite magnetic field H.sub.e of the terrestrial magnetism H and the field H.sub.v. Coordinates (x,y), x.sub.v, y.sub.v) and (x.sub.e, y.sub.e) of signals from the sensor 2 which correspond to coordinates (H.sub.x, H.sub.y), (H.sub.vx, H.sub.vy) and (H.sub.ex, H.sub.ey) and (Hex, Hey) of the magnetic field H, H.sub.v and H.sub.e are shown, in FIG. 4, on a x-y perpendicular coordinate system. Thus, the signals x.sub.e and y.sub.e from the sensor 2 can be represented by: EQU x.sub.e =x.sub.v =KH sin .theta.+x.sub.v ( 3a) EQU y.sub.e =y+y.sub.v =KH cos .theta.+y.sub.v ( 3b)
and an angle obtained from the signals x.sub.3 and y.sub.e according to the equation (2) becomes: EQU tan.sup.- (x.sub.e /y.sub.e) (4)
Thus, a true angle .theta. can not be obtained.
However, since a field H.sub.v is produced by the vehicle 1 itself acting as a permanent magnet and an intensity and direction thereof with respect to the moving direction A of the vehicle 1 are constant, the coordinates (s.sub.v, y.sub.v) of the signal corresponding to the magnetic field H.sub.v shown in FIG. 4 is kept unchanged even if the direction A is changed. Therefore, a locus of the coordinates (x.sub.e, y.sub.e) of the detection signal when the vehicle 1 runs once along a circle route becomes a circle 0.sub.2 having a center point (x.sub.v,y.sub.v) as is clear from the equations (3a) and (3b). Therefore, by obtaining the center coordinates (x.sub.v,y.sub.v) of the circle 0.sub.2 from the detection signals x.sub.e and y.sub.e, a true angle .theta. can be obtained easily from the following equation: EQU .theta.=tan.sup.-1 [( x.sub.e -x.sub.v)/(y.sub.e -y.sub.v)](5)
Japanese Patent Application Laid-open No. 148210/1982 discloses a technique by which a true angle .theta.]is obtained by cancelling out influences of the magnetic field H.sub.v on the basis of the principle mentioned above. In detail, among the detection signals x and y obtained from the terrestrial magnetism sensor 2 when the vehicle 1 circles once, maximum values x.sub.max and y.sub.max and minimum values x.sub.min and y.sub.min in the respective axes of the x-y perpendicular coordinate system are stored and the detection signals x.sub.v and y.sub.v corresponding to the magnetic field H.sub.v is obtained as coordinates of the center of the circular locus 0.sub.2, according to the following equations: EQU x.sub.v =(x.sub.max +x.sub.min)/2 (6a) EQU y.sub.v =(y.sub.max +y.sub.min)/2 (6b)
Therefore, by directing the vehicle 1 along a circular route within a suitable time to obtain the detection signals s.sub.v and y.sub.v corresponding to the magnetic field H.sub.v, it is possible to obtain a true orientation 0 by performing an operation corresponding to equation (5).
However, when the vehicle 1 is, for example, an automobile, it is subjected to vibrations during its movement. Therefore, the magnetic field H.sub.v may vary gradually as shown in FIG. 5, although the variation might be negligible when averaged over, for example, a day. In addition, when the automobile passes a rail road having a d.c. power supply system at a time instance t.sub.o, it may be magnetized by a magnetic field existing between the rails and cables thereof, and thus the intensity and direction of the field H.sub.v may be considerably changed. With such a change of the field H.sub.v, the automobile must circle again to obtain the signals x.sub.v and y.sub.v corresponding to the changed field H.sub.v. This is very difficult to do practically.